Simplify node-js installation from nodesource and a variable for version

[m-anish]

For 7.0, we should think about simplifying the way nodejs is installed. This PR does that:

1. Introduces a variable to set nodejs version
2. Installs from nodesource regardless of platform

[holta]

@jvonau what's the risk of going with #1411 here? (It might even be safer to keep Sugarizer running reliably on Node.js 8.x across all distros...)

[holta]

Clarif: PR #1430 extends this to work with Red Hat-family OS's like CentOS 7 too, i.e. our legacy distros that IIAB has always tried to support informally over the years (but to a lesser degree than Raspbian/Ubuntu/Debian).

[m-anish]

Clarif: PR #1430 extends this to work with Red Hat-family OS's like CentOS 7 too, i.e. our legacy distros that IIAB has always tried to support informally over the years (but to a lesser degree than Raspbian/Ubuntu/Debian).

+1, closing this